I have come across a copy of one of these by chance and have learned thats it's pretty rare...i have read about the japan released ones and that there were only 5000 that were sent out to gran-turismo.com members...but this is american edition and i cant find any information anywhere. If anyone has some info it would be greatly appreciated.
There are many Gran Turismo collectors on this forum who should be able to point you in the right direction but a picture of the disc itself wouldn't do any harm!
Any "Online Public Beta" disc available for NTSC|U/C was related to PlayStation Underground members. Members were randomly selected to test some online games and received the disc by mail. I have no idea of the exact number of how many of these were made, but yes they're uncommon. They are probably only valueable to collectors since these discs usually came on a cardboard/paper slipcase and a sheet of paper with guidelines / joystick setup. Also most of these discs only had the online part available, so they're pratically useless. I own one copy of the japanese GT4 Online Test Disc, and it contains the full game + the online part, plus it comes with on a regular PS2 DVD Case and has a small manual. Don't really know if the GT4 Online Public Beta is the same or just the online part like the rest of the Public Betas.
